The effect of complexity of go no-go decision-making and dimensional overlap of target/non-target stimuli on go no-go CRTs

5.4.2.1 Study 4

In Study 4, based on the results of previous studies (Cooper et al, 1994) it was expected that RTs would become slower from CRTl to CRT2 and from CRT2 to CRT3, as the decision on ‘go’ trials was based on the conjunction of an increasing number of stimulus features (1 to 3). Contrary to expectation, RTs were in fact fastest for CRT3

subjects have to respond to three features of a compound stimulus: size, colour and shape, by responding only to a ‘large yellow circle’ for instance. The finding that RTs for CRT 3 were faster than RTs for CRT2 or CRTl conditions suggests that instead of being the most difficult condition in terms of response selection, CRTS is in fact the easiest. There are two possible reasons for this. First, in CRTS of Study 4 subjects are required to only respond to a single target stimulus e.g. ‘large yellow circle’ as noted above. In contrast, in the CRT2 condition, there are two valid stimuli for example with ‘yellow circle’ as the target there are two possible target stimuli large yellow circles and small yellow circles. In the CRTl condition, there are four possible target stimuli, for example, if one were responding to ‘large’ stimuli the decision to move would be made if the stimulus was a ‘large yellow square’, ‘large purple square’, ‘large yellow circle’, or a ‘large purple circle’, hence there were four valid stimuli. The fact that RTs increase as the number of stimulus or response alternatives increase is a most widely replicated finding, a relationship which is expressed by the Hick-Hyman law (Hick, 1952; Hyman, 1953). Thus, in line with the Hick-Hyman law, RTs were fastest for the CRT3 condition because this involved a single valid stimulus across trials compared to 2 and 4 respectively for CRT2 and CRTl. The second possibility related to the first is that as a result of a single compared to 2 or 4 stimulus alternatives, subjects had fewer examplars of the target ‘go’ stimulus to hold ‘on line’ across trials in a block in CRT3, which would account for the faster RTs. Also, with a single stimulus alternative it is easier to establish and maintain ‘set’ that is a state of preparedness or readiness to respond than with 2 or 4 possible stimulus alternatives.

The design of the go no-go RT of Study 4 was similar to that used by Cooper et al. (1994) in their study with patients with Parkinson’s disease. Unlike the present results, they found that the go no-go CRTs of patients with Parkinson’s disease and the normal

controls increased as stimulus complexity increased. However, there were a number of procedural differences between the Cooper et al., and the present study. First, instead of size as a feature of the compound stimulus. Cooper et al, had coloured circles and squares presented in conjunction with either a high or low-pitched tone. This combination of auditory with visual features may have meant that stimulus discrimination and hence the go no-go decision may have been more difficult in the study of Cooper et al. (1994) than in our study which simply relied on the visual features of compound stimuli. One indication of this would have been if the CRTs were slower in the Cooper et al. study than in the present study. However, the fact that that controls in their study were older than the controls in the present sample, means that a direct comparison of CRTs is not valid. A second important procedural difference is that in the study by Cooper et al. (1994) the response criteria in the CRT conditions changed after the subject made 10 correct responses compared to the present study where response criteria changed after 80 correct responses. Once again, this procedural difference may have made Study 4 easier than the analogous version used by Cooper and colleagues.

In Study 4, another aspect of the procedure used may have inadvertently influenced task difficulty and may help explain why differential effects were not obtained for the patients and controls. On each trial, the specific nature o f the target stimulus, for example Targe yellow circle’ in the CRT3 condition, or ‘yellow circle’ in the CRT2 condition was presented above the stimulus. Provision of this external cue informing subjects of the relevant stimulus dimensions may have reduced the working memory load of Study 4. In light of deficits shown by patients with schizophrenia on tests with a working memory component such as the Wisconsin Card Sorting Test (Nathaniel-James et al., 1996), it would be expected that the patients would show greater deficits in the

absence of such a cue, which would have necessitated relying on working memory across trials in a block.

S.4.2.2 Study 5

For Study 5, as expected, the RTs increased across the three CRT conditions for the controls and patient groups. Thus, as the dimensional overlap of no-go ‘distractor’ stimuli with the target ‘go’ stimulus increased, RTs increased. The only exception was found for the patients with ‘low symptoms’ for whom RTs did not increase between CRTl and CRT2, but did increase for CRTS. The controls were significantly faster than the ‘high symptom’ patients but the two groups did not differ in their RTs across the three conditions. In contrast, the controls and ‘low symptom’ patients differed in terms of the effect of CRT condition on RTs. While the controls had significantly slower RTs in CRT2 and CRTS compared to CRTl, the ‘low symptom’ patients showed no significant difference between CRTl and CRT2 and but were significantly slower on CRTS than the other two conditions. This suggests that for the controls, the greatest relative prolongation of RTs on the go trials was produced when the number of non­ target no-go stimuli increased from one non-target no-go stimulus (always ‘r ’ below fixation) in the CRTl condition to three non-target no-go stimuli in the CRT2 and CRTS conditions. In contrast, for the ‘low symptom’ patients, a significant increase in go RTs was obtained in Study 5 when the non-target no-go stimuli had the greatest dimensional overlap with the target go stimulus and differed from it only in terms of one dimension either identity, case or location; but not when the non-target and target stimuli differed in terms of two (CRT2) or three (CRTS) dimensions.